Preheat the oven to 350°F (175°C). Grease and fully line a 9x5-inch loaf pan with parchment paper.

In a medium bowl, whisk together the flour, granulated sugar, and salt for the streusel. Add the cold butter and vanilla bean paste. Work together with your fingertips until a mixture of fine crumbs and large buttery clusters forms. Refrigerate while preparing the batter.

In a large bowl, combine the granulated sugar and lemon zest. Rub the zest into the sugar with your fingertips until the sugar is fragrant and lightly moistened.

Whisk in the oil, yogurt, vanilla bean paste, lemon extract, lemon juice, and milk until completely smooth.

If making the lemon poppy seed variation, whisk in the poppy seeds.

Sift the flour, cornstarch, baking powder, baking soda, salt, and nutmeg over the wet ingredients. Whisk just until combined and no dry streaks remain. Do not overmix.

Spread just under half of the batter into the prepared loaf pan.

Sprinkle half of the chilled streusel evenly over the batter.

Carefully spread the remaining batter over the streusel until completely covered.

Sprinkle the remaining streusel evenly over the top, leaving plenty of large buttery clusters.

Sprinkle generously with clear coarse sanding sugar before baking.

Bake at 350°F for 55-67 minutes, or possibly longer if your oven runs slower. Check the loaf at 45 minutes to see how the streusel is browning and tent loosely with foil if needed. The loaf is done when the center reaches 205-210°F (96-99°C), the center no longer jiggles, and a tester inserted into the cake portion comes out with moist crumbs but no wet batter.

Cool in the pan for 20 minutes, then carefully lift the loaf onto a wire rack to cool completely.

Whisk together the powdered sugar, lemon juice, 45 g milk, and salt until completely smooth. Add additional milk 1 teaspoon at a time until the glaze ribbons off the whisk but is not watery.

Spoon or pour the glaze generously over the cooled loaf, allowing it to run naturally down the sides.

Allow the glaze to set before slicing.